Recently, surface mounting with increased mounting density has been widely employed as a mounting method in the production of printed wiring boards. Such methods of surface mounting can be classified into (i) a double-sided surface mounting method of joining chip parts with a solder paste, (ii) a mixed mounting method combining surface mounting of chip parts with a solder paste and through-hole mounting of discrete parts, and the like. In each of these mounting methods, a printed wiring board is subjected to multiple rounds of soldering, and therefore, develops a severe thermal history due to repeated exposure to high temperatures.
Exposure to high temperatures can have a negative effect on the copper or copper alloy that constitutes the circuit parts of the printed wiring board due to formation of an oxide coating film on a surface of the copper or copper alloy. Repeated exposure to the high temperatures can accelerated formation of the oxide coating film. Good solderability of the surface of the circuit part cannot be maintained as the coating develops.
In order to protect the copper or copper alloy of a circuit part of such a printed wiring board from air oxidation, a treatment that produces a chemical conversion coating on the surface of the circuit part, using a surface treating composition, is widely used. It is required that the chemical conversion coating be maintained on the circuit part without modification (deterioration) even after the circuit part has received multiple thermal histories, thereby maintaining good solderability.
Eutectic solders comprising a tin-lead alloy have been widely used for joining electronic parts to a printed wiring board and the like. In recent years, however, harmful effects on the human body by lead (Pb) contained in the solder alloy have been recognized, and use of solders free of lead is now required. For this reason, various lead-free solders have been investigated. For example, lead-free solders comprising tin (Sn) as a base metal, having added thereto a metal such as silver (Ag), zinc (Zn), bismuth (Bi), indium (In), antimony (Sb), cobalt (Co), manganese (Mn), nickel (Ni), or copper (Cu), have been proposed.
The conventional Sn—Pb eutectic solder has excellent wettability to a surface of a metal, particularly copper, used in a joining base material, and strongly joins to copper. Therefore, high reliability is achieved in bondability between copper members.
In contrast, lead-free solders typically have poor wettability to a surface of copper, as compared with the conventional Sn—Pb eutectic solder, and therefore have poor solderability. As a result, joining defects, such as the occurrence of voids, are common when lead-free solders are used, which can result in low joint strength.
For this reason, selection of a solder alloy having better solderability and a flux suitable for lead-free soldering is required when lead-free solders are employed. Also, a surface treating composition that can prevent oxidation of a copper or copper alloy surface and that has the properties of improving the wettability of lead-free solder and of allowing good solderability are required.
Many lead-free solders have a high melting point, and therefore the soldering temperature is about 20° C. to 50° C. higher than that of a conventional tin-lead eutectic solder. Therefore, an improved surface treating composition would also be required to form a chemical conversion coating having excellent heat resistance.
Various imidazole compounds are proposed as an effective component of such an improved surface treating composition. For example, Patent Document 1 discloses 2-alkylimidazol compounds such as 2-undecylimidazole, Patent Document 2 discloses 2-arylimidazole compounds such as 2-phenylimidazole and 2-phenyl-4-methylimidazole, Patent Document 3 discloses 2-alkylbenzimidazole compounds such as 2-nonylbenzimidazole, Patent Document 4 discloses 2-aralkylbenzimidazole compounds such as 2-(4-chlorophenylmethyl)benzimidazole, and Patent Document 5 discloses 2-aralkylimidazole compounds such as 2-(4-chlorophenylmethyl)imidazole and 2-(2,4-dichlorophenylmethyl)-4,5-diphenylimidazole. However, when surface treating compositions containing these imidazole compounds were tested, the heat resistance of chemical conversion coatings formed on a copper surface was found to be unsatisfactory. Furthermore, when soldering was performed, the wettability of the solder was found to be insufficient, and good solderability was not obtained. Particularly, when soldering was performed on a copper surface treated with a surface treating composition comprising one of the referenced imidazole compounds and lead-free solder was used in place of tin-lead eutectic solder, acceptable results were not obtained.